double P_drive;    // power needed to drive at speed
double E_drive;    // energy needed to drive at speed
double P_battery;  // power from battery to drive at speed (in additiona to energy from array)
double E_battery;  // energy from battery to drive at speed (in additiona to energy from array)
double B_capacity; // capacity at current power draw.
double DB_charge;  // change in battery charge at current power draw for the time step.
double ddistance;  // distance traveled in the time step.

